Directional vs Omni antennas for mics

[DavidDaMonkey]

We are currently looking into adding some antenna distro to our wireless rig and have come up against the "directional" vs "omnidirectional" question.

My first instinct is to go with directional, as 95% of the action will always take place on the stage. My worry though is that other 5%. The director of the company does occasionally stage pieces in the aisles or out into the lobby even. While the aisles probably wouldn't be a problem, The lobby would put the transmitters directly behind the antennas.

I guess my question is "just how directional are directional antennas?" Specifically we are looking at Sennheiser. Anyone have any thoughts?

 

[Rhett]

I wonder if you could use both. Antenna A being directional and B being omni for example. Let's see what other folk say.

 

[DavidDaMonkey]

I wonder if you could use both. Antenna A being directional and B being omni for example. Let's see what other folk say.

I had the same thought actually, curious to know if it is a good idea or not.

 

[MisterTim]

I wonder if you could use both. Antenna A being directional and B being omni for example. Let's see what other folk say.

No, no, no, no, and no. Bad. Don't do this. Something would probably blow up if you tried to do this with a diversity antenna system. Okay, maybe not, but it wouldn't be good.

As a general rule, you should use omnis. Directional antennas are for longer-distance transmitting and receiving, or for RF rejection if you're having problems from a particular direction. If you're running fine with just built-in antennas now, you'll definitely be fine with omnidirectional antennas.

Also, make sure your antennas are spaced correctly for diversity reception when you install them.

 

[CSCTech]

I would go with Omni if I were you. It's just easier, wouldn't need to worry about any problems. We have never had any reception problems at all with 10 Sennheiser ew 100 wireless mics all with two omni directional antennas each in a 350 seat proscenium, yeah I know, we need a distro

Heh, be happy you get reception in your halls at all xD We lose range the second someone steps out of the room. Yay concrete walls and metal doors!

 

[fredthe]

My feeling is to give wireless mics as much help as you can, and go with directional... depending on the distance, you might still be OK in the lobby.

Directional antennas are typically cardioid, so it depends on how "right behind" the antenna you are. You might see if you can borrow/rent one and try it out first.

 

[museav]

Not knowing where the antennas would be or the facility layout, it's a little hard to say. I will say that to get the benefit of diversity you would typically want similar gain and coverage for each antenna, if you make it where the reception would be primarily limited to one or the other antenna then there is little diversity. In fact if one antenna run is much longer than the other you sometimes may intentionally run a longer cable than necessary or use higher loss cable for the shorter run in order to compensate and keep both antennas with similar losses.

 

[Chris15]

Question 1 is where are you looking for these antennas to be sited? FOH or on stage?

I'd tend to go for directional antennas, the extra gain can be your friend as well as the rejection of spurious noise. If the aisles are within the antenna's beamwidth, then you'll be fine and even if you are starting to push the limits of that beamwidth, the transmitter being closer will probably make up that loss.

The lobby... Probably you'll make it into a dead zone RF wise. There's then 2 schools of thought to deal with that. 1 is to say to your director well we can't make it work, please adjust your performance accordingly or 2 is to solve the problem if and when it happens. Move the receivers to the lobby for that production and run audio cable back is an option. Doing a cable swap to the back of the receivers for a portion of a production is quite doable (speaking as one who has done it at a pro level to get around a similar problem). So you might keep the receivers in the rack but put omnis onto the presumably small number of channels you need in the lobby, all of these are options...

Brad, they make attenuators to deal with those problems... or amplifiers...

 

[Rhett]

Since we're on the topic, I have some questions for the RF experts:

1) Diversity - Can you explain how this technology works. Its my current understanding that it simply favors the stronger signal. But I also read something about how there's processing that compares the two signals to discern reflections.

2) From a FOH position, what would be the down side of pointing a directional antenna toward the stage for Antenna A, and using an omni to cover the room for Antenna B? More specifically, are there RF consequences and what benefit is lost without diversity?

3) What would be the down side of pointing one directional antenna toward the stage and the other towards the back lobby. I'm thinking only when one needs RF in the lobby, the rest of the time, the two directions would point toward the stage, hopefully achieving the benefit of diversity.

4) Is there a system out that uses more than 2 antennas. Say, triversity?

Thanks for entertaining my curiosities.

Rhett

 

[Chris15]

1) Diversity - Can you explain how this technology works. Its my current understanding that it simply favors the stronger signal. But I also read something about how there's processing that compares the two signals to discern reflections.

Generally speaking, yes the system will switch to the stronger signal at a given time. There are some systems that will do fancier things, but if you need that functionality you'll be needing someone who knows how it works

2) From a FOH position, what would be the down side of pointing a directional antenna toward the stage for Antenna A, and using an omni to cover the room for Antenna B? More specifically, are there RF consequences and what benefit is lost without diversity?

The directional antenna will provide RF gain, for most LDPAs (batwings), in the order of 10dB. So the A signal will be arriving 10dB hotter than the B signal and so the selectivity normally associated with picking the stringer signal is lost, the receiver will just sit on antenna A because it has the stronger signal until it gets a dead spot that drops the RF enough that B is stronger. Basically it don't work as good... Lack of diversity means when you get multipath interference or obstructions between transmitter and receiver you lose the signal totally. The idea with diversity is that, setup properly, you should be able to "see" at least one antenna more often than you can see a single antenna. This is why spatial diversity is a good thing if you can get it without undue inconvenience - a couple of metres between antennas is ideal.

3) What would be the down side of pointing one directional antenna toward the stage and the other towards the back lobby. I'm thinking only when one needs RF in the lobby, the rest of the time, the two directions would point toward the stage, hopefully achieving the benefit of diversity.

One antenna will be blind at any given time. Thus no diversity and dead spots. Note too that anywhere between the 2 "zones" will be mostly dead...

4) Is there a system out that uses more than 2 antennas. Say, triversity?

Not that is in common usage. 2 is enough for normal situations where you are using diversity for what it is designed for...

However to achieve what you are talking about with multiple coverage areas, yes there are a variety of options available. One is to have a second set of receivers, appearing as separate audio channels and located appropriately. Another involves combining antennas to feed receivers, refer page 10 [pdf].

Some resources:
Shure Americas | Publications
Lectrosonics - Free Wireless Guide

 

[museav]

1) Diversity - Can you explain how this technology works. Its my current understanding that it simply favors the stronger signal. But I also read something about how there's processing that compares the two signals to discern reflections.

There are multiple type of diversity and it is not always clear which is implemented. Antenna diversity typically is a single receiver that looks at the incoming signals from two antennas and constantly switches to the antenna producing the strongest signal. True diversity uses two separate receivers and compares the output signal of those, switching to the one it considers better.

2) From a FOH position, what would be the down side of pointing a directional antenna toward the stage for Antenna A, and using an omni to cover the room for Antenna B? More specifically, are there RF consequences and what benefit is lost without diversity?

As Chris said, the idea behind spatial diversity, that is having the two antennas physically separated, is that conditions that might cause dropouts or interference as the transmitter moves around are much less likely to affect both antennas simultaneously.

With the Sennheiser antennas the A 2003-UHF directional paddle has a greater than 14dB front-to-back ratio but only an approximately +4dB gain compared to the omni A 1031 U. To get close to the +10dB gain Chris noted you'd have to go to the circularly polarized A 5000 CP, which has about a +8 to +9.5dB gain and a narrow 40 degree half power beam width.

 

[mbenonis]

Lots of good discussion here, especially from Chris and Brad. My opinion is that you should definitely go with directional antennas pointed at the stage. The reality is that the null won't be all that great, and if you're in the lobby you're probably close enough to the antennas for things to work no matter where you are. Directional antennas are not like lasers--rather, they just favor signals from one direction over another, and that's only really significant with really weak signals. In a typical theatre, your signals will be much stronger and will probably be received from all angles, at least strongly enougnto work fine.

Don't buy powered antennas, by the way. Preamplifiers are the number one cause of weird anomalies in wireless mic systems. Essentially, one strong signal can cause the preamplifier to fold up and distort, killing performance. Be sure to buy passive antennas instead.

PS, sorry for the late reply, I've been at a wireless conference all week and away from the computer for a while.

 

[mbenonis]

Since we're on the topic, I have some questions for the RF experts:

1) Diversity - Can you explain how this technology works. Its my current understanding that it simply favors the stronger signal. But I also read something about how there's processing that compares the two signals to discern reflections.

As stated above, diversity receivers typically switch between two signals based on information such as received signal strength. Some receivers switch at the antennna input, some at IF, and some at baseband with two complete receivers. Some systems can do fancier diversity, including switching between two frequencies with two separate transmitters on the talent.

Reflections aren't typically a concern in our systems, as you won't really hear them.

2) From a FOH position, what would be the down side of pointing a directional antenna toward the stage for Antenna A, and using an omni to cover the room for Antenna B? More specifically, are there RF consequences and what benefit is lost without diversity?

No real RF consequences aside from possibly worse reception on the stage. It would work, though it would be better to just use two LPDA antennas pointing at the stage.

3) What would be the down side of pointing one directional antenna toward the stage and the other towards the back lobby. I'm thinking only when one needs RF in the lobby, the rest of the time, the two directions would point toward the stage, hopefully achieving the benefit of diversity.

Once again, you lose diversity on the stage, where your performers are the vast majority of the time. Also, like I said in my other post, thenlobbynwillmprobably get into the antennas anyway, either from the back or from reflections...

4) Is there a system out that uses more than 2 antennas. Say, triversity?

Not that I know of. Lectrosonics can fo frequency diversity, or true receiver diversity, but not more than two antennas.

 

[TimmyP1955]

We decided to start cheap antenna wise, and upgrade if we had problems. We bought 8x EW100G3 and a pair of ASA1. Racked and wired it all up, mounted a pair of BNC sockets to the top of the (metal) rack, and popped on a pair of the tiny whip antennas that came with the receivers. It works perfectly. (Though in fairness, outside RF is exceedingly low in our venue. Great for wireless, bad if you have an AT&T cell phone or you want to watch broadcast TV.)